I went to Airfix after a quick Google search to see if anyone else had similar problems.
Original kit manufacturers tend to have a parts replacement service e.g. Airfix or Revell. I think the importer for Hasegawa kits does a similar thing.
I found that you can send details of exactly what part (kit, sprue number etc.) you need through the contact form on the Airfix website so, I gave that a go and it worked - very quickly too!
I don't know for sure, but I expect Mr. Models wouldn't have been able to replace the part without breaking open a whole kit - and I don't know if this sort of thing is refund/exchange territory. I'd started the kit before realising anyway.